Various approaches have been proposed and demonstrated for the probing and stabilization of PRTR, including the use of a Fabry–Pérot (FP) cavity [1]}, a frequency spectrum analyzer (FSA) [2]} and Schottky diode detectors (SDD) [3]}. The FP method extracts the PRTR by scanning the cavity length for sequential power detection, while the FSA and SSD methods rely on beating the Raman beams with an additional reference beam and the induced beatnotes are used for servo locking the PRTR.
However, the FP method fails to provide real-time monitoring and the FSA method is limited by the acquisition rate and maximum 10 Hz bandwidth for PRTR control. Finally, the SDD method provides several tens of kHz bandwidth at the cost of complicated optical and electronic processing.
The latency or the complexity of the above mentioned methods prevent the applications of electro-optic modulated Raman lasers in AIs for field applications.
An alternative and intuitive solution is to spatially separate the laser pair and directly monitor and fast control their power ratio. However, considering the frequency gap to be around 6.8 GHz in Rb, it is difficult for traditional dispersion devices such as gratings and prisms to spatially separate Raman laser pair with large enough distance for probing. Following this intuition, we focused on a type of spectral dispersion etalon named the virtually imaged phased array (VIPA), which can separate the spectrum in sub-GHz regime [4]}, [5]}.
Due to its simple structure, polarization insensitivity and high dispersion, VIPA has been initially used in wavelength division multiplexing [6]}, optical communication [7]}, pulse shaping [8]}, tomography [9]}, etc.
